Goal:
  
  A : Type
  P : nat → A → A → Type
  Q : nat → A → A → Type
  i : nat
  hd : A
  hd' : A
  tl : list A
  p : P i hd hd'
  HPQ : ∀ (i : nat) (x y : A), P i x y → ∑ z : A, Q i x z × Q i y z
  x : A
  q : Q i hd x
  q0 : Q i hd' x
  ============================
   (OnOne2i Q i (hd' :: tl) (x :: tl))


DEBUG:Going to execute:
match goal with
           | |- ?X1 → ?X2 => intro
           | |- ¬ _ => unfold not at 1; intro
           end
Level 0: Backtrace:

No matching clauses for match.